In addition to the dystopian themes, the novel almost takes you back in history to the Wild Wild West. There are "pioneers" and "outlaws" and the people live on underwater "homesteads." All in all, it was a really interesting concept.
Anyway, the main character, Ty, meets a girl named Gemma who lives above the sea - those people are called "Topsiders". Gemma's brother has lived undersea for a while, and she is trying to find him. She enlists Ty's help. Meanwhile, Ty and his family/neighbors are fighting off the invasion of a group of outlaws called the Seablite Gang. Ty and Gemma soon discover that her brother is NOT who she thought he was, and Ty has to use his "Dark Gift" - an almost magical byproduct of living under the sea for so long - to help both Gemma and his family.
I really loved all that the book had to offer - a mix of dystopian themes, elements of fantasy (the "Dark Gifts"), history, mystery, and suspense. I'd recommend it to anyone looking for something different, especially boys. It's also a series; I can't wait to read the next one, Rip Tide.